144 ANNUAL OF SCIENTIFIC DISCOVERY. 



he gives the plate the shortest possible immersion in the nitrate 

 bath, keeping it in motion from the first, to get rid rapidly of the 

 greasy, streaky appearance of the plate. The solution running 

 evenly over the film, without streaks or oily-looking lines, which 

 is generally regarded as the indication of sufficient immersion, is, 

 in reality, no test of the conversion of the salts in the collodion 

 film into salts of silver; it merely indicates that the alcohol and 

 ether in the film have become thoroughly mixed with the aqueous 

 solution, and that the mutual repulsion has ceased. Under ordi- 

 nary circumstances, however, by the time this is thoroughly 

 effected, the mutual decomposition of the iodides originally in the 

 collodion and the nitrate of silver, and the formation of iodide of 

 silver and a nitrate of potash, or other base, is also complete. 

 With bromides, this operation is not so rapidly completed ; if, 

 therefore, a collodion film containing a large portion of bromide 

 be immersed and kept in motion so as rapidly to get rid of greasi- 

 ness, and then removed after a very brief immersion, the film will 

 contain a large portion of the bromide, say, of cadmium or am- 

 monium, which remains undecomposed, and is not converted 

 into bromide of silver. In this fact lies the safety of the plate for 

 long exposures. The free nitrate of silver which would other- 

 wise be crystallizing on the surface of the film, or, by the concen- 

 tration of the solution caused by evaporation, acquiring a readier 

 tendency to abnormal reduction now performs a different oifice ; 

 being in contact with the unconverted bromide of cadmium or 

 ammonium, it is decomposed by it, and aids in the formation of 

 bromide of silver in the film. Instead of being made stronger by 

 evaporation of water, the free nitrate is made weaker by the loss 

 of the silver which combines with the bromine, whilst the nitric 

 acid, combining with the base which leaves the bromine, produces 

 an innocuous, or possibly in some cases a hygroscopic, and there- 

 fore beneficial salt. It will thus be readily seen how the use of a 

 large portion of bromide and a very short immersion of the plate 

 in the nitrate bath tend to prevent the stains of crystallization or 

 of reduction consequent on long exposure in warm weather. The 

 mode in which the effect in question is secured in the case de- 

 scribed may possibly suggest an explanation of the general action 

 of bromides as aids to clean negatives. It is probable in most 

 cases where a freely bromized collodion is employed, and the plate 

 kept in the nitrate bath the usual two or three minutes, that some 

 portion of unconverted bromide remains in the film, and that the 

 formation of bromide of silver goes on after the plate leaves the 

 bath, the bromide of silver being formed at the expense of the free 

 nitrate on the film, which is thus much weakened. As the use of 

 a weak solution of nitrate silver, at times secured by redipping 

 the plate in a weak bath, is known to be conducive to cleanli- 

 ness, the weakening of the free nitrate by the formation of bro- 

 mide of silver may also be a source of the cleanliness well known 

 as an accompaniment of the use of bromides. 



The amount of bromide in collodion for very long exposures 

 may vary from two grains to two and a half. Any soluble bro- 



